It is often useful to sense entry into (or exit from) a room or other building space by a person, or simply presence of a person in the space. This may be for security purposes, or to detect human presence in or near the room in order to activate lights, open doors, or activate heat for the space. Existing entry and occupancy detectors (hereafter “motion detectors”) use at least five ways to detect passage or impending passage of a person through a door, or presence of a person in a room.
Some systems use infra red (IR) sensing with a sensor that detects the changes in IR strength that a heat source such as a human body causes. Similar to this are detectors that rely on changes to ambient visible light that a moving human body causes to sense presence of that body.
Other systems rely on sounds or air pressure changes that an approaching person or animal might cause. Such audio-based systems often signal presence of a human when in fact, the sound could be from a completely extraneous source such as plumbing or a passing auto.
These are passive systems, in that they rely on some existing energy level whose detectable changes related to movement or other activity of the body that indicate human presence or passage. Think of the automatic door openers in supermarkets as one example. To some extent, this passive detection is advantageous, say when such a system properly detects continued presence of a person in the room. However, such systems may generate many false positives and false negatives.
Other systems are active systems having a source of some type of beam or energy whose changes in level caused by human movement can be detected by a sensing element. Such systems typically use a dedicated source producing IR, ultrasonic (sound), or microwave signals. In general such sensing elements use interruption of or other change in the IR, ultrasound, or microwave signal as the basis for detecting entry, exit, or presence of a body.
A common problem with these active systems is that often the signal source and the sensor element are mounted on the surface of the wall. This may be unappealing esthetically for some, and in any case lends itself to inadvertent damage or vandalism.
One such active system now available uses a microwave radiation source, typically a laser diode, for the energy whose change is detectable to indicate nearby motion. Microwave radiation is sometimes defined as electromagnetic radiation in the 0.3-300 ghz range. For purposes of this document, the term “microwave radiation” includes any electromagnetic radiation that can penetrate non-metallic sheets or layers with relative ease, but is substantially attenuated by metallic sheets or layers.
A preferred unit now available comprises a single combined occupancy detector module holding both the source and the sensing element. The sensing element relies on changes in the backscatter and reflection of a portion of the radiation to indicate a person's approach. For entry and exit detection, such a module may be positioned near a door so that an approaching person will be detected, and an automatic opener activated. Such a unit has a sensitivity adjustment to allow for differing requirements in different applications. One such unit uses a 5.8 ghz radiation source.
In many ways, an active system based on microwave energy is ideal for sensing movement, since one can be confident that the only source for the microwave energy is that associated with the system. The components of such a system are relatively inexpensive and reliable.
The disadvantage of an active microwave-based system is that the energy from the microwave source will propagate over a wide area. Microwave radiation is not easily focused by the source, so it can easily reach into a neighboring space. Movement in such a neighboring space is then sensed, creating false positives. There are possible solutions to this problem but these may be difficult to implement in the field. For example, sensitivity adjustment of the unit may eliminate most of these false positives, but may also then produce some false negatives, that is where indications of movement should be but are not detected.
Current microwave Doppler signal type of occupancy sensors are inherently subject to uncontrolled coverage and cannot effectively operate only when actually needed within a room where installed. The ability of the microwave signal to pass through the typical stud and gypsum board constructed wall regardless of location results in nuisance light activations. This characteristic has eliminated the market interest for using this sensing method in most of the occupancy sensor room lighting installations.
These microwave sensors have adjustable sensitivity but since the microwave coverage cannot be limited to a single room, or a well-defined footprint, nuisance tripping occurs. Present designs are successful only if the sensor is centrally located within a symmetrical dimensioned room. Even then, the sensitivity must be adjusted for the room boundaries. These considerations limit the installations where this type of occupancy sensor can be used.
Architects and interior designers consider exposed occupancy sensors to be very unattractive architecturally. A completely concealed sensor will be preferred in many cases.
Building codes include the use of occupancy sensors within rooms as a “green” energy saving measure to replace manual switches. Also, occupancy sensors remove the chances of unsupervised access to public area lighting. This means at least one occupancy sensor device per room, and in larger rooms, such as open office areas many occupancy sensors will be required to be evenly spaced throughout the ceiling area for proper coverage to detect the presence of any movement/activity within the room to continue to maintain power to the room lighting fixtures.